This invention relates to control systems.
The invention is particularly, though not exclusively, concerned with fluid or hydraulic control systems for use in conjunction with an actuator linked to the control surfaces (such as, for example, the ailerons or rudder) of an aircraft.
It is important in many applications of control systems to provide protection against failure or malfunction in the system. This is, of course, especially important for applications in aircraft. It is common practice in, for example, hydraulic control systems to provide such protection by using two hydraulic circuits to control movement of a single actuator. The two circuits are operated simultaneously, malfunction being detected by comparing the behaviour of the two circuits. This may be done by providing feedback to the two circuits such that an excessively high or low output force from only one circuit results in an opposing output from the other circuit to counteract it. Although this arrangement may help reduce the effects of a malfunction, it is difficult to maintain control of the system since in general it is not possible to identify within which of the two circuits the malfunction occurs.
Some previous arrangements, alternatively, have had two hydraulic circuits only one circuit being operated at any time, the other circuit being switched to control the actuator if a malfunction is detected. This arrangement is preferable since it gives full control of the system once the switching has taken place. With this arrangement, however, it can take a relatively long time for the switching to be effected since it is necessary to open and close various hydraulic valves in order to switch out one circuit and to switch in the other. This can be a severe disadvantage, and this is especially so where the system is used in conjunction with aircraft control surfaces since the aircraft may be out of control during this time.